1. Field of the Invention
The invention relates to an apparatus to generate a seek direction detecting signal for an optical pickup to determine the relative position of a center of an optical spot focused on an optical disk with respect to a center of a disk track in a RAM-type optical disk having a land/groove structure, and more particularly, to an apparatus to generate a seek direction detecting signal for an optical pickup using a main beam and a sub-beam having a predetermined aberration in a radial direction.
2. Description of the Related Art
In general, an optical pickup apparatus optically records or reproduces information of an optical disk, such as a RAM type disk, which is used as a recording medium to record/reproduce information. A conventional optical pickup apparatus comprises a light source to emit laser light, an objective lens to converge light emitted from the light source on an optical disk to form an optical spot on the optical disk, an optical detector to receive light reflected from the optical disk and to detect an information signal and an error signal, and a signal processing portion to process the detected signals.
Referring to FIGS. 1 and 2, a conventional apparatus to generate a seek direction detecting signal for an optical pickup comprises a grating (not shown) to illuminate both a track and adjacent tracks at the same time so as to record/reproduce information, an optical detector, and a signal processing portion.
As shown in FIG. 1, a spiral track having a land (L) and groove (G) structure is formed on an optical disk 1, and information marks 1a are formed on the lands (L) and grooves (G). When information is recorded on or reproduced from the optical disk 1, light beams diverged by the grating are focused on the disk 1. That is, a main beam BM is focused on the track to record/reproduce the information marks 1a, and first and second sub-beams BS1 and BS2, which respectively precede and follow the main beam BM by a predetermined time, are focused on tracks of the optical disk 1 to be ±½ track pitch off the main beam BM in a radial direction of the optical disk 1.
As shown in FIG. 2, the optical detector includes a main optical detector 2a to receive the main beam BM reflected from the optical disk 1, and first and second optical sub-detectors 2b and 2c to receive the first and second sub-beams BS1 and BS2, respectively. The main optical detector 2a comprises two separate plates A and B that are separated in the radial direction of the optical disk 1 and independently receive light. In addition, the first and second optical sub-detectors 2b and 2c comprise two separated plates C and D, and two separated plates E and F also separated in the radial direction of the optical disk 1, respectively.
The signal processing portion comprises a plurality of differential amplifiers 3, 4, 5, and 6 to differentiate and amplify the signals detected by the main optical detector 2a, and the first and second optical sub-detectors 2b and 2c to detect a track error signal (TES) and a track cross signal (TCS), a tracking control unit 7 to receive the detected TES and TCS and to output a tracking control signal, and an objective lens driving portion 8 to actuate an objective lens (not shown) according to the input tracking control signal.
The TES uses a push-pull signal Md output from the main optical detector 2a. The TCS uses a difference signal S1d–S2d of push-pull signals S1d and S2d respectively output from the first and second optical sub-detectors 2b and 2c, and the push-pull signal Md. Since the first and second sub-beams BS1 and BS2 are disposed ±½ track pitch off the main beam BM, and the push-pull signals S1d and S2d have phase differences of ±90° with respect to the push-pull signal Md, the TCS can be detected using the phase differences.
While the conventional apparatus to generate the seek direction detecting signal for an optical pickup has an advantage in that the TCS can be detected in a relatively simple manner using the three beams, there is a problem in that, since the first and second sub-beams BS1 and B2 are disposed to precede and follow the main beam BM, cross erasures occur on adjacent track signals when a RAM-type optical disk is used.